It mostly appear on youtube, but it can be done on facebook as well. A thumbshunter is a person who writes a comment on youtube and the only reason the person writes the comment, is to get thumbs up from other users.
Another frequent example is a person who tries to write something funny to get as many thumbs up/likes as possible, and it usually ends up in not being funny at all.

a person is a thumbshunter if he writes in a youtube video: "thumps up﻿ if you restart this song over and over" or something like that.
it can be twisted with a comment which is hating on those who pressed the dislike button. There's an example in Miley Cyrus - Party in the USA: 20.599 people is not having a party.
X: "why is he writing that, it isnt even funny" Y: "dude, he's just a thumbshunter, dont mind him"